More generally, the term agent can be used to mean anything which can act within an environment. In addition to its use in the field of Multi-Agent Systems, the term will often pop up in other fields or languages, such as software engineering and UML. When discussing agents, there are some other terms that often appear as well:
- World: AKA the environment, means the collection of all objects under consideration, IE, everything that is available to interact with. Often it also includes a set of rules, for example, the laws of physics, that all objects must obey.
- Object: Anything in the world that can be acted upon.
- Agent: Anything in the world that can initiate interactions, acts independently, and has motivations.
The important thing here is that the difference between an agent and an object (even though an agent quite often is also an object...) is that an agent acts freely, an object can only act when acted upon.
Suppose Joe, Jane, and a ball are inside a cubical room with no exits. In this case, the agents are Joe and Jane (assuming they are alive and awake). The objects are Joe, Jane, the ball, and the walls of the room. (Joe and Jane are objects in this example, as I will show.) The world is Joe, Jane, the Ball, the walls, the space within the walls, and the laws of physics. These concepts are so basic that we generally don't have words for them outside of specialty fields.
Joe can kick the ball. The ball responds to being kicked, but it'd be most odd if the ball acted on its own. If it did, we would obviously speculate that there was a third agent present (maybe a ghost). Joe can also kick Jane, and Jane will respond according to the laws of physics (respond as an object), as well as responding according to her own motivations (responding as an agent). If, for this example, we ignore the overly cumbersome process of describing sound wave propagation, etc, we will say that the verbal reprimand that Jane gives Joe for kicking her is agent-to-agent communication, which does not involve the world or any object within.
It is feasible, and practical, to consider and study a pure agent system, where the world is empty except for agents who may communicate with each other. You may visualize this as disembodied philosophers chatting endlessly in limbo using telepathy.
Understanding "worlds", "agents", and "objects" by these definitions feels surprisingly natural once they are grokked. This is because, in fact, it is likely the most basic thing that a new mind must grasp, possibly the first steps of mental development that we undergo as newborns.
As was said before, the definitions of these terms vary from person to person and from context to context. Or, different words might be used altogtether. However, if you understand the concepts here you are off to a good start.